The present invention relates to an exhaust gas-purifying device for use in an internal combustion engine.
As a catalyzer capable of simultaneously reducing the amount of three harmful components (HC, CO, and NO.sub.x) in exhaust gas, a three-way catalyzer is known. As illustrated in FIG. 1 (a), when the air-fuel ratio A/F is equal to an approximately stoichiometric air-fuel ratio, the efficiency R of HC, CO, and NO.sub.x conversion in the three-way catalyzer reaches a maximum, and the range of the air-fuel ratio, in which a conversion efficiency of more than 80 percent can be obtained, is a narrow range of about 0.06 A/F unit. The range of the air-fuel ratio, in which a conversion efficiency of more than 80 percent can be obtained, is normally called a window W. Consequently, in order to simultaneously reduce the amount of the three harmful components in the exhaust gas by using a three-way catalyzer, it is necessary to continuously maintain the air-fuel ratio within the window W, which is a narrow range. To this end, in a conventional exhaust gas-purifying device, an oxygen concentration detector, capable of detecting whether the air-fuel ratio is larger or smaller than the stoichiometric air-fuel ratio, is arranged in the exhaust passage of the engine, and the air-fuel ratio is controlled so that it becomes equal to the air-fuel ratio within the window W on the basis of the output signal of the oxygen concentration detector. However, in a conventional exhaust gas-purifying device using such an oxygen concentration detector, since an expensive oxygen concentration detector and an expensive electronic control unit for controlling the air-fuel ratio are necessary, a problem occurs in that the manufacturing cast of the exhaust gas-purifying device is increased.
However, recently, as disclosed in SAE paper No. 760201 and U.S. Pat. No. 4,024,706, the function of the three-way catalyzer has gradually been clarified, and it has been proven that the three-way catalyzer has an oxygen storage function. That is, when the air-fuel ratio is on the lean side of the stoichiometric air-fuel ratio, the three-way catalyzer accepts oxygen from NO.sub.x and reduces NO.sub.x, and the three-way catalyzer stores therein the accepted oxygen. On the other hand, when the air-fuel ratio is on the rich side of the stoichiometric air-fuel ratio, the three-way catalyzer releases the stored oxygen therefrom and oxidizes CO and HC. Therefore, in a case where the air-fuel ratio fluctuates relative to a given reference air-fuel ratio so that the air-fuel ratio is alternately on the lean side and the rich side of the stoichiometric air-fuel ratio, even if the reference air-fuel ratio is offset from the stoichiometric air-fuel ratio, the reducing reaction of NO.sub.x and the oxydizing reaction of CO and HC are promoted due to the above-mentioned oxygen storage function, thus making it possible to obtain a high conversion efficiency. FIG. 1(b) illustrates a window W.sub.0 of the reference air-fuel ratio A/F in a case where the air-fuel ratio fluctuates at a frequency of 1 Hz by a .+-.0.1 A/F unit relative to the reference air-fuel ratio. From FIGS. 1(a) and 1(b), it will be understood that if the air-fuel ratio fluctuates at a fixed frequency, the width of the window is increased. This means that if the air-fuel ratio fluctuates at a fixed frequency, a high conversion efficiency can be obtained even if the reference air-fuel ratio is somewhat offset from the stoichiometric air-fuel ratio. On the other hand, if the frequency of fluctuation of the air-fuel ratio becomes low, that is, if the time period of fluctuation of the air-fuel ratio becomes long, since the oxygen storage capacity of the three-way catalyzer becomes saturated, the oxygen storage ability of the three-way catalyzer is reduced. As a result, the conversion efficiency of the three-way catalyzer is reduced. This is clearly illustrated in FIG. 1(c ). In FIG. 1(c), the ordinate R indicates conversion efficiency, and the abscissa F indicates the frequency of fluctuation of the air-fuel ratio. In addition, if the range of fluctuation of the air-fuel ratio becomes small, since it is impossible to fluctuate the air-fuel ratio so that it is alternately on the lean side and the rich side of the stoichiometric air-fuel ratio, the width of the window becomes narrow. Consequently, it will be understood that an optimum frequency and an optimum range of fluctuation of the air-fuel ratio are present for increasing the width of the window W.
As mentioned above, if the range of fluctuation of the air-fuel ratio relative to the reference air-fuel ratio and the frequency of fluctuation of the air-fuel ratio are suitably determined, the width of the window W is increased. Therefore, even if the reference air-fuel ratio fluctuates relative to the stoichiometric air-fuel ratio, a high conversion efficiency can be obtained. This means that if a fuel supply system, in which the range of fluctuation of the reference air-fuel ratio is narrow, is used, a high conversion efficiency can be obtained without using feedback control based on the output signal of the oxygen concentration detector. Of course, if a fuel injection system is used as such a fuel supply system, it is possible to reduce the range of fluctuation of the reference air-fuel ratio. However, since a fuel injection system is very expensive, a problem occurs in that the manufacturing cost of the engine is increased. Consequently, in order to reduce the manufacturing cost of the engine, it is necessary to use a carburetor. However, in a conventional fixed venturi-type carburetor, the range of fluctuation of the reference air-fuel ratio is wide, and in a conventional variable venturi-type carburetor, the reference air-fuel ratio fluctuates considerably at the time of acceleration and in response to a change in the temperature of the engine. Consequently, it is difficult to obtain a high conversion efficiency by using either a conventional fixed venturi-type carburetor or a conventional variable venturi-type carburetor.